Of course, Mario wouldn't need to speak at all to be a test subject in Aperture Science's extensive Testing Facility (where science happens!). His go kart racing experience won't help him overcome GLaDOS's maniacal obstacles, but his ability to become flat like a piece of paper might.
Not to mention the fact that the dude can jump higher than anyone in the tri-state area. Strap a pair of Chell's "Long Fall Boots" on his fat feet and he'll be unstoppable. He won't even need a portal gun to reach the outside world, and he'll be taking the Companion Cube with him this time.